Setup unit tests

[tlestang]

This is a PR into branch minimal_cmake, to have a smaller diff and make review easier. It should either be merged before #20 is, or alternatively this branch will be rabased on the tip of master

This adds a placeholder unit test, built with CMake and pFUnit.
Test is built and run each time commits are pushed to this repo.

The test code itself is defined in tests/ as a free-form Fortran file. This file is parsed when the pFUnit CMake module is invoked:

# tests/CmakeLists.txt
# ...
add_pfunit_ctest(tests TEST_SOURCES ${test_src})

Test executable is built typically as

mkdir build && cd build
cmake ..
make tests # create test executable in build/tests/

The GitHub actions workflow unit_tests.yml is very similar to the one introduced in #18 , but it build and caches pFUnit in additon to openmpi, and uses cmake to build the test exe.